(I'd be very sorry to hear this is a stupid question, but I've done already some considerable googling and can't make a sense out of it.)

If I try to develop some software that uses RPC, I do not only find out that the RPC Portmapper is not yet running on my system, but also I can't find nor start it. While I could not find any trace about this, this is a clear sign for me that Apple decided to either withdraw support for the good only ONC RPC, or they switched to something else, that I cannot find tho.

Apologies around your question don't help. Losing more time reading apologies isn't a solution for losing time on trivial/stupid question. If you have a question, let's just hope it's not stupid (doesn't seem stupid to me).
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Denys SéguretAug 20 '12 at 11:00

FWIW on the page you linked to, where it lists the command for OS X as launchctl start com.apple.portmap it also lists the command for BSD as /usr/sbin/rpcbind, which is there in Mtn Lion and would probably explain why launchctl start rpcbind works.